1. Field of the Invention
The invention relates generally to focused ultrasound systems and, more particularly, to systems and methods for sensing and locating discontinuities and disturbances in the energy path of an ultrasound beam in a focused ultrasound system.
2. Background
Thermal energy, such as high intensity focused ultrasonic waves (acoustic waves with a frequency greater than about 20 kilohertz), may be used to therapeutically treat internal tissue regions within a patient. For example, ultrasonic waves may be used to ablate tumors, thereby eliminating the need for invasive surgery. For this purpose, piezoelectric transducers driven by electric signals to produce ultrasonic energy have been suggested that may be placed external to the patient but in close proximity to the tissue to be ablated. The transducer is geometrically shaped and positioned such that the ultrasonic energy is focused in a “focal zone” corresponding to a target tissue region within the patient, heating the target tissue region until the tissue is necrosed. The transducer may be sequentially focused and activated at a number of focal zones in close proximity to one another. This series of “sonications” is used to cause coagulation necrosis of an entire tissue structure, such as a tumor, of a desired size and shape.
A spherical cap transducer array, such as that disclosed in U.S. Pat. No. 4,865,042 issued to Umemura et al., has been suggested for this purpose. This spherical cap transducer array includes a plurality of concentric rings disposed on a curved surface having a radius of curvature defining a portion of a sphere. The concentric rings generally have equal surface areas and may also be divided circumferentially into a plurality of curved transducer elements or sectors, creating a sector-vortex array. The individual transducer elements are driven by radio frequency (RF) electrical signals at the single frequency, but offset in phase and amplitude. In particular, the phase and amplitude of the respective transducer element drive signals may be controlled so as to focus the emitted ultrasonic energy at a desired “focal distance,” i.e., the distance from the transducer to the center of the focal zone and provide a desired energy level in the target tissue region.
While the transducer is located external to the patient, it must be in direct contact and tightly coupled with a media that efficiently transmits the high frequency ultrasound waves. For example, the transducer can be positioned in a liquid bath that is capable of efficient transmission of the ultrasound waves. The patient's body must also be wetted and tightly coupled to the transmission media in order to ensure an optimal acoustic wave transmission path from the transducer to the focal zone. If there are any interruptions in continuity (i.e., “discontinuities”) along the path, they will generate reflections of the ultrasound waves. Such reflections can reduce the efficiency of the treatment, cause damage to the transducer, and misdirect the ultrasound waves to tissue outside the treatment zone. For example, air pockets or bubbles, can be trapped in the transmission media between the patient and the transducer. Also, the portion of the energy path inside the patient may contain bone or a blood vessel with an air bubble in it, or the sonication process can overheat the targeted tissue causing gas bubbles to form therein.
In other words, it is not uncommon for the transmission path to contain reflective discontinuities or other disturbances. It would be desirable to be able to sense whether any such disturbances in the acoustic energy transmission path exist prior to initiating a sonication, so that corrective action can be taken to avoid harmful reflections of the ultrasound waves.